Hybrid laser-induced breakdown spectroscopy system

ABSTRACT

A LIBS system to detect constituent elements of interest within a sample from plasma light resulting from irradiation of this sample is presented. The LIBS system has a hybrid configuration which provides both a low-resolution spectrum of the plasma light covering a broad spectral range, and a high-resolution spectrum of the same plasma light over a narrow spectral range centered on a spectral line or feature of a constituent element of interest of the sample. In some implementations, the LIBS system has a portable design and can perform onsite sample analyses.

TECHNICAL FIELD

The technical field generally relates to material analysis and moreparticularly concerns a hybrid LIBS system.

BACKGROUND

Laser-induced breakdown spectroscopy (LIBS) is a well-known technique toretrieve elemental information from a given sample. A typical LIBSmeasurement is performed as follows: a short laser pulse is sent andfocused onto a sample surface; the surface is rapidly heated by thelaser pulse, part of the material is vaporized, and the gas istransformed into plasma, the plasma composition being representative ofthe sample's elemental content; excited electrons in the plasmaeventually return to the ground state of their associated atoms as theplasma cools, and the radiative electron recombination emits photonswith discrete energies allowed by their associated atoms energy levels;and the emitted photons are collected and sent in a spectrometer toproduce optical emission spectra. The spectral distribution of thespectra (intensity versus wavelength) is linked to the elementalcomposition of the plasma, hence to the elemental composition of thesample. For example, see U.S. Pat. No. 6,008,897 and references citedtherein.

LIBS systems known in the art typically fall within one of three types:

-   -   1—Laboratory LIBS systems: Systems of this type are built to        have optimal performance in terms of laser energy, high power of        resolution and sensitivity. This typically involves the use of        bulky components for the laser source, spectrometer and        detector. The working distance from the sample to the focusing        lens (or what is known in the LIBS art as lens-to-sample        distance LSD) typically varies from 25-50 cm to 2 m.    -   2—Industrial LIBS system: In this category, performance is also        key in order to satisfy the industrial requirements, and bulky        components can also be used as volume is not a critical issue.        Additional requirements are the robustness of the overall system        and low cost of ownership. Working distance or LSD varies from        30-50 cm to 2 m and is usually greater than 50 cm.    -   3—Handheld systems (less than 2 kg): systems of this type        require the use of less bulky components than the previous        types. However, the performance of components such as lasers,        spectrometers and detectors is often related to their volume.        For instance, reducing the size of a laser comes at the cost of        less energy per pulse, and a less bulky spectrometer means less        power of resolution and using a lower sensitivity detector. In        addition, the working distance or LSD is in the order of few cm        (generally less than 10 cm).

There remains a need in the field for a LIBS system that could combinehigher sensitivity comparable to laboratory or industrial systems, withthe portability of handheld systems.

SUMMARY

In accordance with one aspect, there is provided a Laser-InducedBreakdown Spectroscopy (LIBS) system to detect a constituent element ofinterest within a sample.

The LIBS system includes a pulsed laser source generating light pulsesapt to create a plasma upon irradiating said sample. The LIBS systemfurther includes an element detection assembly and a broadband detectionassembly. The element detection assembly includes a high-resolutionspectrometer having a narrowband spectral range covering a spectralfeature of the constituent element of interest, whereas the broadbanddetection assembly includes a low-resolution spectrometer having abroadband spectral range.

The LIBS system further includes a probe head transportable by a user toa sample site and having a probing interface configured to irradiate thesample with the light pulses and collect resulting plasma light. Probeoptics optically coupling the probing interface with the pulsed lasersource, the low-resolution spectrometer and the high-resolutionspectrometer are also provided. The probe optics define a first outputlight path directing a narrowband spectral portion of the plasma lightencompassing said spectral feature of the constituent element ofinterest to the high-resolution spectrometer, and a second output lightpath directing a broadband spectral portion of said plasma light to thelow-resolution spectrometer.

In some implementations, the probe optics include an upstream dichroicfilter centered on a wavelength of the light pulses, the upstreamdichroic filter being positioned to respectively direct:

-   -   the light pulses from the laser source towards the probing        interface; and    -   the plasma light from the probing interface towards the element        detection assembly and broadband detection assembly.

In some implementations, the probe optics further include a scanningmirror assembly provided between the upstream dichroic filter and theprobing interface.

In some implementations, the probing interface comprises a transparentwindow.

In some implementations, the pulsed laser source is mounted within theprobe head.

In some implementations, the probe optics include a downstream dichroicfilter centered on the spectral feature of the constituent element ofinterest and disposed to separate the plasma light into said narrowbandand broadband spectral portions.

In some implementations, the probe optics are mounted within the probehead.

In some implementations, the LIBS system further includes a firstoptical fiber link having a fiber input disposed to receive thenarrowband spectral portion of the plasma light from the probe opticsand a fiber output connected to the high-resolution spectrometer, and asecond optical fiber link having a fiber input disposed to receive thebroadband spectral portion of the plasma light from the probe optics anda fiber output connected to the low-resolution spectrometer.

In some implementations, the element detection assembly may include aphotomultiplier detector or an avalanche photodiode coupled to an outputof the high-resolution spectrometer.

In some implementations, the broadband detection assembly may include aCCD camera coupled to an output of the low-resolution spectrometer.

In some implementations, the high-resolution spectrometer is based on aCzerny-Turner configuration using cascaded primary and secondarygratings without intervening optics therebetween.

In some implementations, the low-resolution spectrometer is based on afolded or unfolded Czerny-Turner configuration comprising a planargrating.

In some implementations, the low-resolution spectrometer comprises aconcave grating.

In some implementations, the LIBS system, further includes a mobilehousing enclosing therein the element detection assembly and thebroadband detection assembly, a power supply unit enclosed within saidmobile housing, and wire connectors providing electrical and opticalcommunication between the mobile housing and the probe head.

In accordance with another aspect, there is also provided aLaser-Induced Breakdown Spectroscopy (LIBS) system to detect aconstituent element of interest within a sample from plasma lightresulting from irradiation of said sample. The LIBS system includes anelement detection assembly comprising a high-resolution spectrometerhaving a narrowband spectral range covering a spectral feature of theconstituent element of interest, and a broadband detection assemblycomprising a low-resolution spectrometer having a broadband spectralrange.

In some implementations, the high-resolution spectrometer is based on aCzerny-Turner configuration using cascaded primary and secondarygratings without intervening optics therebetween.

In some implementations, the element detection assembly includes anavalanche photodiode, a photomultiplier tube, a single-photon avalanchediode (SPAD) or a Silicon photomultiplier detector (SiPM) coupled to anoutput of the high-resolution spectrometer.

In some implementations, the low-resolution spectrometer is based on afolded or unfolded Czerny-Turner configuration comprising a planargrating. Alternatively, the low-resolution spectrometer may include aconcave grating.

In some implementations, the broadband detection assembly includes a CCDcamera coupled to an output of the low-resolution spectrometer.

In some implementations, the LIBS system further includes probe opticsdefining a first output light path directing a narrowband spectralportion of the plasma light encompassing said spectral feature of theconstituent element of interest to the high-resolution spectrometer, anda second output light path directing a broadband spectral portion ofsaid plasma light to the low-resolution spectrometer. The probe opticsmay include a downstream dichroic filter centered on the spectralfeature of the constituent element of interest and disposed to separatethe plasma light into said narrowband and broadband spectral portions.The LIBS system may further include a first optical fiber link having afiber input disposed to receive the narrowband spectral portion of theplasma light from the probe optics and a fiber output connected to thehigh-resolution spectrometer, and a second optical fiber link having afiber input disposed to receive the broadband spectral portion of theplasma light from the probe optics and a fiber output connected to thelow-resolution spectrometer.

In some implementations, the LIBS system may be optically coupled to apulsed laser source generating light pulses apt to create a plasma uponirradiating said sample. The LIBS system may further include a probehead transportable by a user to a sample site and having a probinginterface configured to irradiate the sample with the light pulses andcollect resulting plasma light. The pulsed laser source may be mountedwithin the probe head.

In some implementations, the probe optics may include an upstreamdichroic filter centered on a wavelength of the light pulses, theupstream dichroic filter being housed in the probe head and positionedto respectively direct:

-   -   the light pulses from the laser source towards the probing        interface; and    -   the plasma light from the probing interface towards the element        detection assembly and broadband detection assembly.

In some implementations, the LIBS system further includes a mobilehousing enclosing therein the element detection assembly and thebroadband detection assembly, a power supply unit enclosed within saidmobile housing, and wire connectors providing electrical and opticalcommunication between the mobile housing and the probe head.

Other features and advantages of the invention will be better understoodupon reading of embodiments thereof with reference to the appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a LIBS system according to oneembodiment.

FIG. 2 is a schematic representation of a high-resolution spectrometerdesign according to one embodiment.

FIGS. 3A to 3C are schematic representation of three examples of designsof a low-resolution spectrometer.

FIG. 4 is an illustrated of a portable design for a LIBS systemaccording to one embodiment, shown in use.

FIG. 5A is a side elevation view of the exterior of a probe headaccording to one embodiment; FIGS. 5B and 5C are side elevation viewsfrom two different directions of the interior of the probe head of FIG.5A.

FIG. 6 is a graph showing a spectrum obtained with a broadband detectionassembly according to one embodiment of a LIBS system; FIG. 6A is anenlarged view of the spectrum of FIG. 6 in the range between 267.0 nmand 268.0 nm.

FIG. 7 is a graph showing a spectrum obtained with an element detectionassembly according to one embodiment of a LIBS system.

FIG. 8 is a calibration curve obtained using an elemental detectionassembly according to one embodiment of a LIBS system.

DETAILED DESCRIPTION

Embodiments described herein generally concern a LIBS system to detect aconstituent element of interest within a sample.

As readily understood by those skilled in the art, LIBS generally relieson the use of a repetitively-fired laser source to emit intense andshort pulses of light that are used to ablate/vaporize matter from asample target. The interaction of the light pulses with the vaporizedmatter creates a plasma plume, which in turn radiates light. Theanalysis of the plasma-emitted light brings qualitative and quantitativeinformation on the nature and concentration of the constituent elementalcomponents of the target. More specifically, the qualitative andquantitative data related to the elemental components of the target isobtained from the processing and analysis of the spectral signature ofthe plasma-emitted light.

In a typical LIBS configuration, the light emitted by the plasma isoptically collected and brought into a spectrometer, whose function isto extract the spectral information contained in the plasma-emittedlight. The output of the spectrometer consists of a spectrum (in theform of a two-dimensional profile representing the light intensity vs.optical wavelength), which is characteristic of the collected light. Thespectral distribution is recorded by means of a detector (often a lineor 2-D camera).

The spectral profile provided by the spectrometer is made up of acollection of spectral lines. Each of these lines is related to anelement present in the plasma plume. The elements found in the plasmacome from the ablated/vaporized matter from the target and from theambient gas, if any. The analysis of the spectral lines providesinformation on the nature of the elements in the plasma as well as theirconcentration.

In some implementations, the LIBS systems presented herein have a hybridconfiguration which provides both a low-resolution spectrum of theplasma light covering a broad spectral range, and a high-resolutionspectrum of the same plasma light over a narrow spectral range centeredon a spectral line or feature of a constituent element of interest ofthe sample.

LIBS systems according to the present description may be useful in avariety of context where an elemental analysis of a sample is desired,such as soils or organic products analysis, the evaluation of mineralsand other samples from the mining industry, material science andthin-film analysis, pharmaceutical products monitoring, material sortingand recycling, archeology and cultural artifacts studies, etc.

In some implementations, the present LIBS system may be of use in thecontext of mining, in particular gold mining. Gold mines are veryimportant economic assets for many countries in the world. However,mining industries are facing increasing decisional challenges associatedwith lower grade ore and complexity of mineralization with higherimpurity levels, which imply more frequent sample analyses in theproduction process. Mining sample analyses performed using conventionaltechniques typically involve wait times of at least 24 hours, causingproduction delays on the mining or exploration sites and thus increasingthe operating and production costs.

In order to address these issues, the mining industry would benefit fromthe measure of precious metal concentration in real time and on siteduring the different exploration and mining production stages. In thecase of gold, the ability to measure an average concentration down toabout 1 ppm is desired. Existing technology, such as infraredspectroscopy, allows determining the mineralogy of the rock samples(quartz, pyrite, chalcopyrite, sphalerite, arsenopyrite, etc.), but theelemental composition is out of reach with this technique. Furthermore,X-ray fluorescence has been used successfully for determining theconcentration of some basic metals such as copper, zinc and nickel; itis however inadequate for quantifying gold concentration, because of thelow sensitivity and poor limits of detection. Additionally, the goldspectral line used in x-ray fluorescence suffers from interference witha strong zinc line which compromises the sensitivity of this techniquefor the determination of gold concentration.

LIBS technology is a suitable candidate for providing the desiredanalysis of gold samples. However, the detection of gold in rocks byprior art LIBS setups or instrumentation at such low concentrationlevels requires the use of high resolution spectrometer and highlysensitive ICCD detector which are bulky, costly and not robust; as aresult of these drawbacks, the prior art LIBS instrumentation is notwell adapted for onsite and harsh mining environment.

Advantageously, embodiments of LIBS systems described herein can providea fast method for measuring the content of gold, and identifying thematrix in which gold is embedded. In addition, such embodiments providea portable instrument that can be brought onsite for fast analysiswithout sample preparation.

LIBS System

Referring to FIG. 1, there is schematically illustrated a LIBS system 20according to one embodiment. It will be readily understood that theconfiguration illustrated and described herein is shown by way ofexample only and is in no way meant as limitative to the invention.

In some implementations, the illustrated LIBS system can be designed ina compact portable arrangement and can be brought to a sample 22 foranalysis. Features of such an arrangement are described and explainedfurther below.

The LIBS system 20 may first include a pulsed laser source 24 generatinglight pulses 26 apt to create a plasma 23 upon irradiating the sample22, according to the LIBS process described above. As well known in theart, the measurement sensitivity depends on the laser beam fluence(defined as the ratio corresponding to the laser pulse energy divided bythe area of the beam spot) at the target surface. For instance, toachieve the ablation of the target material and create a plasma, aminimum (threshold) value of the fluence must be reached. Furthermore,the sensitivity is a function of the radiant flux emitted by the plasma;for a given fluence, the larger the plasma size (that is, the larger thebeam spot size), the higher the total radiant flux which can becollected by the system. By way of example, the pulsed laser source 24may be embodied by a flash lamp-pumped (FP) or diode-pumped solid-state(DPSS) laser source with active Q-switching, or the like. The lightpulses may having a pulse energy from a few mJ to a few hundreds of mJ;a spot size (diameter) from a few 10s μm to 1 mm; and a repetition ratefrom a few Hz to 100 Hz. In accordance with some implementations, theduration of the laser pulses is short, for example in the nanosecondregime. The full width at half-maximum (FWHM) of the pulses may forexample be within the range of a few nanoseconds. Therefore, in thisregime, the plasma light emission begins just after the laser pulsefiring; it then grows, decays and finally disappears after a certainperiod of time, referred to as the plasma lifetime.

Hybrid Detection Scheme

In accordance with some implementations, the LIBS system 20 has a hybridconfiguration including two different detection schemes, enabled by twoseparate detection assemblies: an element detection assembly 58, and abroadband detection assembly 66. The element detection assembly 58includes a high-resolution spectrometer 100 having a narrowband spectralrange covering a spectral feature of the constituent element ofinterest, whereas the broadband detection assembly 66 includes alow-resolution spectrometer 200 having a broadband spectral range.

In the context of the present description, the term “resolution” inmeant to refer to the spectral resolution of the correspondingspectrometer, typically defined as the minimum wavelength differencebetween two wavelengths that can be resolved unambiguously. Theexpression “high-resolution” is meant to refer to a resolutionsufficient to allow the identification of a spectral feature of theconstituent element of interest for a given application. The expression“narrowband” is meant to refer to a spectral bandwidth broad enough tocover the spectral feature of interest while being small enough todistinguish this spectral feature. By contrast, the expression“low-resolution” and “broadband” are meant to refer to a resolution andspectral range allowing an overview of the spectral contents of theplasma light without necessarily permitting identification of allindividual lines. Furthermore, it will be readily understood that theterminology explained above is used herein in relatively, that is, todistinguish the different components of the LIBS system from each otherwithout imparting limits on the scope of protection.

In some embodiments the high-resolution spectrometer 100 of the elementdetection assembly 58 may be based on the so-called Czerny-Turnerconfiguration or Czerny-Turner spectrometer, a dominant design ofspectrometers used in LIBS analysis. In such a configuration, thereceived plasma light is transferred to an array detector via an opticalpath that involves one or more dispersing elements. In other variants,the high-resolution spectrometer may be based on other designs known inthe art, such as for example an echelle spectrometer.

Referring to FIG. 2, there is shown an exemplary design for the elementdetection assembly 58. In this example, the high-resolution spectrometer100 is based on a Czerny-Turner design using cascaded gratings. Such adesign is shown in provisional patent application No. 62/662,468 filedon Apr. 25, 2018 and entitled “High resolution and high throughputspectrometer”, the entire contents of which is incorporated herein byreference.

In the particular implementation of FIG. 2, the spectrometer 100includes an input slit 126 through which a light beam 122 to be analyzedis received, followed by one or more collimating lenses 128. The inputslit 126 creates a point-type source from the incoming light, and thelight beam 122 is therefore spatially divergent upon entering thespectrometer. The collimating lens 128 is disposed across the path ofthe diverging light beam 122 and aligns its composing beamlets along aparallel direction, thereby collimating the light beam 122. Eachcollimating lens 128 may be embodied by a cylindrical lens or by aspherical singlet lens, a multi-element spherical lens assembly (such asa combination of plano-convex and meniscus lenses, or an achromaticdoublet), by a non-spherical singlet lens (such as a best-form oraspheric lens), or the like.

The spectrometer 100 further includes a primary diffraction grating 130on which the light beam 122 impinges. In the illustrated variant, theprimary diffraction grating 130 is disposed immediately downstream thecollimating lens 128, without intervening optics. In the illustratedimplementation, the light beam 122 impinges on the primary diffractiongrating 130 at normal incidence.

As known in the art, light at normal incidence on the primarydiffraction grating 130 will be diffracted according to the so-calledbasic grating equation. Preferably, the primary diffraction grating 130is designed such that light at wavelength of interest is diffractedwithin the −1 and +1 diffraction orders of the grating, defining twoprimary diffracted beams 131 and 131′.

The spectrometer 100 further includes two planar secondary diffractiongrating 136 and 136′ positioned in a path of the primary diffractedbeams 131 and 131′, preferably at normal incidence. Each secondarydiffraction grating 136 and 136′ diffracts the corresponding primarydiffracted light beam 131 and 131′ into a twice diffracted beam 137,137′. In this embodiment, the primary and secondary diffraction gratings130 and 136, 136′ are disposed in a cascade without intervening opticstherebetween. The provision of a pair of secondary diffraction gratings136, 136′ and corresponding branches can advantageously provide theparallel and simultaneous analysis of two different spectral featureswithin a same spectral band of the light beam 122.

The spectrometer may further include one or more imaging lens 144, 144′disposed in the path of each twice diffracted beam 137, 137′. Thespectrometer 100 therefore provides as output two focused light beams oflimited spectral bandwidth in which different wavelengths are spatiallyseparated. As will be noticed, in the illustrated variant the secondarydiffraction gratings 136, 136′ 62 are positioned so as to reflect thecorresponding twice diffracted beam 137, 137′ rearwardly of the primarygrating 130, in a cross-beam configuration. Such a configuration canprovide a long focal length within an optimized compact form factor.

The element detection assembly 58 further includes a photodetector 152,152′ apt to provide a spectrogram of the output light of both branchesof the spectrometer 100. Each photodetector 152 may for example beembodied by an avalanche photodiode, a photomultiplier tube, asingle-photon avalanche diode (SPAD), a Silicon photomultiplier detector(SiPM). The photodetector may also consist in a linear ortwo-dimensional array of individually addressable SPADs or SiPMs; such acombination of detectors would allow to record a portion of the spectrallight distribution found in the spectrometer image plane. Thephotodetector 152, 152′ may be spectrally resolved. In the illustratedvariant, mechanisms providing a fine tuning of the wavelength on eachphotodetector 152, 152 may be provided. Such a mechanism may for examplebe embodied by a wavelength tuning refractive plate 154,154′ used intransmission, whose angular position may be accurately controlled usingminiature stepping motors with encoders (not shown).

Characteristics and relative positions of optical components of thespectrometer 100, define the range of wavelengths the spectrometer 100is able to consider in the analysis. While such spectrometer can beapplied for high-quality analysis, due to physical characteristics ofthe optical components of the spectrometer required to reach asufficient range of wavelengths, the optical path defined by the opticalcomponents of the spectrometer 100 cannot be made arbitrarily short. Inparticular, the operation of the diffraction element 106 typicallyrequires a certain minimum length for the optical path. In other words,the minimum size of the portable analyser employing the spectrometer 100is limited due to the length of the optical path. On the other hand,having a portable analyser device of as small size as possible would bepreferred to make the handling of the analyser device more convenientfor the user and also to enable using the analyser device in narrowspaces. The configuration described above and other equivalents designcan advantageously be helpful in minimizing the footprint of thespectrometer 100, favoring portability.

Referring to FIGS. 3A to 3C, there is shown an example of a broadbanddetection assembly 66, including the low-resolution spectrometer 200 anda detector, for example a CCD line camera 206 as known in the art. Insome embodiments the low-resolution spectrometer may also be based on aCzerny-Turner configuration, for example a single-grating design such asknown in the art. By way of example, such a Czerny-Turner configurationmay be of the unfolded type such as shown in FIG. 3A, and may include aninput slit 202, a plane grating 204, a collimating spherical mirror 208and a focusing spherical mirror 210. Referring to FIG. 3B, theCzerny-Turner configuration may also be of the folded/crossed type, inwhich the light paths intersect; this design allows a more compact formfactor than its unfolded counterpart. Such a configuration includes aninput slit 212, a plane grating 214, a collimating spherical mirror 218and a focusing spherical mirror 220. In other embodiments, thelow-resolution spectrometer may be based on the use of a concavegrating. This design relies on a fewer number of optical components thanthe Czerny-Turner approach, since the beam collimating and imagingfunctionalities are both performed by the grating itself, owing to itsconcaveness. Referring to FIG. 3C, there is shown a typical basicconcave grating design comprises an input slit 222 and a concave grating224.

In accordance with some implementations, the high-resolutionspectrometer 100, the low-resolution spectrometer 200 or both areoperated in a time-gated regime. As is known to those skilled in theart, the temporal behaviour of the LIBS plasma-emitted light iscorrelated to the evolution of the plasma temperature and the electronicdensity. In an initial phase of the plasma lifetime, the plasma light isdominated by a “white light” continuum that has little intensityvariation as a function of wavelength. This light is caused bybremsstrahlung and recombination radiation from the plasma, as freeelectrons and ions recombine in the cooling plasma. If the plasma lightis integrated over the entire life-time of the plasma, this continuumlight can seriously interfere with the detection of weaker emissionsfrom minor and trace elements in the plasma. For this reason, LIBSmeasurements are usually carried out using time-resolved detection. Inthis way the strong background light from the initial phase can beremoved from the measurements by turning the detector on after thisbackground light has significantly subsided in intensity, but atomicemissions are still present. Relevant parameters for time-resolveddetection generally include t_(d), the time between plasma formation andthe start of the observation of the plasma light, and t_(b), the timeperiod over which the light is recorded.

By selecting a proper time delay t_(d) between the onset of the lightpulse and the signal acquisition window, the optimum contrast betweenthe intensity the spectral lines of interest and the signal backgroundcan be achieved. This increases the dynamic range of the measurement,which in turn contributes to maximize the sensitivity of the techniqueand to achieve lower values for the limit of detection (LOD).

When performing time-resolved measurements, the gated spectral signal isacquired at each laser shot (or laser pulse). To achieve time-resolvedmeasurements, a CCD camera equipped with an image intensifier (ICCD) isused as detector. In this configuration, the image intensifier has twofunctions: it acts as a very fast optical shutter (typically with sub-nsrise and fall times), therefore allowing the selection of relevantgating parameters t_(d) and t_(b) with high accuracy and shot-to-shotreproducibility; and owing to its adjustable internal gain, it allowsmatching/optimizing the dynamic range of the input signal intensity withthe camera's CCD sensor.

In some implementations, delayed signal acquisition (t_(d)) may also beperformed using low cost line cameras such as those equipping somecompact spectrometers. However, these detectors have substantiallimitations related to the acquisition gate width (t_(b)), which in somecases cannot be set below a given value (e.g. the ms range).

Probe Optics

Referring back to FIG. 1 and with additional reference to FIGS. 5A and5B, as will be readily understood by one skilled in the art, the LIBSsystem 20 may include probe optics 28 directing, shaping, focusing,collecting or otherwise acting on light travelling within the system.

The probe optics may define a probing light path 29 generally directingthe light pulses 26 from the pulsed light source 24 to the sample 22 andcollecting the resulting plasma light 25. A transparent window orequivalent structure can define a probing interface 50 through whichlight exists and enters the LIBS system 20. The probe optics 28 mayfurther define a first output light path 72 directing a narrowbandspectral portion 53 of the plasma light 25 encompassing the spectralfeature of the constituent element of interest to the high-resolutionspectrometer 100, and a second output light path 74 directing abroadband spectral portion 59 of the plasma light 25 to thelow-resolution spectrometer 200. The probe optics 28 therefore opticallycouples the probing interface 50 with the pulsed laser source 24, thelow-resolution spectrometer 200 and the high-resolution spectrometer100.

In the illustrated embodiment, the probe optics 28 include, along theprobing light path, a laser beam attenuator 30 positioned downstream theoutput of the pulsed laser source 24, for example embodied by apolarizer 32 at a 45 degrees angle with respect to the propagationdirection of the light pulses 26 and positioned between a halfwave plate34 and a quarterwave plate 36. The probe optics 28 next include a laserbeam expander 38, here illustrated as lenses 40. The probe optics 28 mayfurther include a focusing and imaging lens 44, and a scanning mirrorassembly 46. The scanning mirror assembly 46 is for example embodied bya pair of pivoting mirrors 48 a, 48 b which can be jointly operated tospatially scan the light pulses 26 over the sample 22 through thetransparent window 50, as is well known in the art. It will be readilyunderstood that the laser beam attenuator 30, laser beam expander 38focusing and imaging lens 44 and scanning mirror assembly 46 are typicalcomponents well known in the art of optics and that a variety ofdifferent components or configurations could alternatively be used, aswell known to those skilled in this art.

Still referring to the configuration of FIG. 1, the probe optics 28include an upstream dichroic filter 42 provided in the path of the lightpulses 26, for example positioned between laser beam expander 38 and thefocusing imaging lens 44. As known to those skilled in the art, dichroicfilters are optical components having a birefringence designed to splitincoming light according to spectral content. In the illustratedexample, the upstream dichroic filter 42 is a bandpass filter centeredon the wavelength of the light pulses 26; accordingly, the light pulses26 are transmitted through the upstream dichroic filter 42, whereas theplasma light 25 at other wavelengths incident thereon is reflected. Theupstream dichroic filter 42 is positioned to respectively direct thelight pulses 26 from the laser source 24 towards the probing interface50, and the plasma light 25 from the probing interface 50 towards theelement detection assembly 58 and broadband detection assembly 66. Byway of example, the upstream dichroic filter 42 may be disposed at a 45°angle with respect to the common propagation axis of the light pulses 26and plasma light 25. Of course, in other configurations a notch filtercould be used and/or the upstream dichroic filter 42 may be arranged totransmit the plasma light 25 and reflect the laser pulses 26.

The probe optics 28 next include a downstream dichroic filter 52centered on the spectral feature of the constituent element of interest.The downstream dichroic filter is disposed to separate the plasma light25 into the narrowband and broadband spectral portions 53 and 59. In theillustrated configuration, the downstream dichroic filter 52 is a notchfilter reflecting the narrowband spectral portion 53 and transmittingthrough the broadband spectral portion 59. Of course, in otherconfigurations a bandpass filter could be used and/or the downstreamdichroic filter 52 may be arranged to transmit the narrowband spectralportion 53 and reflect the broadband spectral portion 59

Along the first output light path 72, the LIBS system 20 may include afirst optical fiber link 56 having a fiber input 55 disposed to receivethe narrowband spectral portion 53 of the plasma light from the probeoptics 28, and a fiber output 57 connected to the high-resolutionspectrometer 100. A first focusing lens 54 may be provided upstream thefirst optical fiber link 56 to focus the narrowband spectral portion 53of the plasma light onto the fiber input 55. Of course, numerous otherconfigurations are possible using any number of optical components aswell known in the art.

Along the second output light path 74, the LIBS system 20 may furtherinclude a second optical fiber link 64 having a fiber input 63 disposedto receive the broadband spectral portion 59 of the plasma light fromthe probe optics 28, and a fiber output 65 connected to thelow-resolution spectrometer 200. In the illustrated configuration, awideband mirror 60 redirects the broadband spectral portion 59 in adirection parallel to the propagation direction of the narrowbandspectral portion 53 and a second focusing lens 62 may be providedupstream the second optical fiber link 56 to focus the broadbandspectral portion 59 of the plasma light onto the fiber input 63. Again,numerous other configurations are possible using any number of opticalcomponents as well known in the art.

Portable Design

Referring to FIGS. 4, 5A, 5B and 5C, in some implementation the LIBSsystem 20 described herein may be embodied in a portable design. By“portable” it is understood that an operator or user may carry all thecomponents of the system to a site of a sample to perform the LIBSanalysis on-site. It will be further understood that the portable designof the present LIBS system 20 does not necessarily involve that thesystem can be handheld, i.e. fit in an operator's hand, although in someimplementations at least some components of the LIBS system 20 may besmall enough to be handheld.

In the illustrated embodiment, the LIBS system includes probe head 70transportable by a user or operator to a sample site. The probe head 70includes a probing interface as defined above, i.e. configured toirradiate the sample with the light pulses and collect resulting plasmalight. The pulsed laser source may be mounted within the probe head 70,although in some embodiment it may be part of a separate structureoptically connected to the probe head via optical fiber. The probeoptics, or at least some components thereof, may also be mounted withinthe probe head 70.

Referring more particularly to FIGS. 5A, 5B and 5C, an exampleconceptual design of a probe head 70 is illustrated. In this design, theprobe head houses all of the components of the probe optics 28 asdescribed above. Of course, other configurations could be implemented.In some variants, the probe head 70 may be mounted on a swiveling basepod 68 or similar structure facilitating its handling.

Referring back to FIG. 4, the LIBS system 20 further includes a mobilehousing 80 in which are enclosed the element detection assembly and thebroadband detection assembly. Other components may also be provided inthe mobile housing 80, such as for example a power supply unit 82 forproviding electrical power to active components of the system. Wireconnectors 84 can provide electrical and optical communication betweenthe mobile housing 80 and the probe head 70. In the illustratedembodiment, the mobile housing 80 is the size of a suitcase, althoughdifferent form factors and sizes may be considered depending on thenature of the components housed therein. Depending on the intendedcontext of use, the probe head 70 and mobile unit 80 may be made ofrugged materials suitable to the environment of the sample site and aptto protect the components therein.

Example

Referring to FIGS. 6 to 8, examples of data that can be obtained usingLIBS systems such as described herein are presented.

FIGS. 6 and 6A illustrate the spectra obtained on a quartz chloritematrix. The full spectrum obtained through the broadband detectionassembly is shown, and a window illustrates the high-resolution spectrumobtained through the element detection assembly, showing the dependenceof gold versus concentration. Furthermore, the full spectrum allows todraw quantitative information on the concentration of several elementscontained in the matrix, such as Si, Mg, Ca, Na, etc., which may bepresent at the % level. This can be achieved by performing univariateanalysis of the spectral data, using appropriate spectral lines found inthe full spectrum. One can also deploy chemometric (multivariate)analysis methods and algorithms, such as the Principal ComponentsAnalysis PCA, and apply them to the spectral data extracted from thefull spectrum. Such methods can be used to draw information pertainingto the mineralogy of the sample being probed, as known in the art.

FIG. 7 shows the narrowband spectrum obtained through thehigh-resolution spectrometer, centered on the 267.59 nm gold spectralline. As already mentioned above, univariate analysis can also beperformed using the 267.59 nm line in order to obtain the traceconcentrations of gold in the matrix. Moreover, information contained inthe full spectrum, such as selected spectral background data or theenergy density measured within a given spectral range, can be used todetermine the proper univariate calibration parameters to be applied tothe high-resolution data, as a function of the actual mineralogicalmatrix encountered.

FIG. 8 illustrates a calibration curve obtained with the high-resolutionspectrometer described herein, again using univariate processing of thedata obtained from the quartz chlorite reference gold samples.

Of course, numerous modifications could be made to the embodimentsdescribed above without departing from the scope of the invention asdefined in the appended claims.

1. A Laser-Induced Breakdown Spectroscopy (LIBS) system to detect aconstituent element of interest within a sample, said LIBS systemcomprising: a pulsed laser source generating light pulses apt to createa plasma upon irradiating said sample; an element detection assemblycomprising a high-resolution spectrometer having a narrowband spectralrange covering a spectral feature of the constituent element ofinterest; a broadband detection assembly comprising a low-resolutionspectrometer having a broadband spectral range; a probe headtransportable by a user to a sample site and having a probing interfaceconfigured to irradiate the sample with the light pulses and collectresulting plasma light; probe optics optically coupling the probinginterface with the pulsed laser source, the low-resolution spectrometerand the high-resolution spectrometer, the probe optics defining a firstoutput light path directing a narrowband spectral portion of the plasmalight encompassing said spectral feature of the constituent element ofinterest to the high-resolution spectrometer, and a second output lightpath directing a broadband spectral portion of said plasma light to thelow-resolution spectrometer.
 2. The LIBS system according to claim 1,wherein the probe optics comprises an upstream dichroic filter centeredon a wavelength of the light pulses, the upstream dichroic filter beingpositioned to respectively direct: the light pulses from the lasersource towards the probing interface; and the plasma light from theprobing interface towards the element detection assembly and broadbanddetection assembly.
 3. The LIBS system according to claim 2, wherein theprobe optics further comprises a scanning mirror assembly providedbetween the upstream dichroic filter and the probing interface.
 4. TheLIBS system according to any one of claim 1, 2 or 3, wherein the probinginterface comprises a transparent window.
 5. The LIBS system accordingto any one of claims 1 to 4, wherein the pulsed laser source is mountedwithin the probe head.
 6. The LIBS system according to any one of claims1 to 5, wherein the probe optics comprises a downstream dichroic filtercentered on the spectral feature of the constituent element of interestand disposed to separate the plasma light into said narrowband andbroadband spectral portions.
 7. The LIBS system according to any one ofclaims 1 to 6, wherein the probe optics are mounted within the probehead.
 8. The LIBS system according to any one of claims 1 to 7, furthercomprising: a first optical fiber link having a fiber input disposed toreceive the narrowband spectral portion of the plasma light from theprobe optics and a fiber output connected to the high-resolutionspectrometer; and a second optical fiber link having a fiber inputdisposed to receive the broadband spectral portion of the plasma lightfrom the probe optics and a fiber output connected to the low-resolutionspectrometer.
 9. The LIBS system according to claim 8, wherein theelement detection assembly comprises a photomultiplier detector or anavalanche photodiode coupled to an output of the high-resolutionspectrometer.
 10. The LIBS system according to any one of claims 1 to 9,wherein the broadband detection assembly comprises a CCD camera coupledto an output of the low-resolution spectrometer.
 11. The LIBS systemaccording to any one of claims 1 to 10, wherein the high-resolutionspectrometer is based on a Czerny-Turner configuration using cascadedprimary and secondary gratings without intervening optics therebetween.12. The LIBS system according to any one of claims 1 to 11, wherein thelow-resolution spectrometer is based on a folded or unfoldedCzerny-Turner configuration comprising a planar grating.
 13. The LIBSsystem according to any one of claims 1 to 11, wherein thelow-resolution spectrometer comprises a concave grating.
 14. The LIBSsystem according to any one of claims 1 to 13, further comprising: amobile housing enclosing therein the element detection assembly and thebroadband detection assembly; a power supply unit enclosed within saidmobile housing; and wire connectors providing electrical and opticalcommunication between the mobile housing and the probe head.
 15. ALaser-Induced Breakdown Spectroscopy (LIBS) system to detect aconstituent element of interest within a sample from plasma lightresulting from irradiation of said sample, said LIBS system comprising:an element detection assembly comprising a high-resolution spectrometerhaving a narrowband spectral range covering a spectral feature of theconstituent element of interest; a broadband detection assemblycomprising a low-resolution spectrometer having a broadband spectralrange.
 16. The LIBS system according to claim 15, wherein thehigh-resolution spectrometer is based on a Czerny-Turner configurationusing cascaded primary and secondary gratings without intervening opticstherebetween.
 17. The LIBS system according to claim 15 or 16, whereinthe element detection assembly comprises an avalanche photodiode, aphotomultiplier tube, a single-photon avalanche diode (SPAD) or aSilicon photomultiplier detector (SiPM) coupled to an output of thehigh-resolution spectrometer.
 18. The LIBS system according to any oneof claims 15 to 17, wherein the low-resolution spectrometer is based ona folded or unfolded Czerny-Turner configuration comprising a planargrating.
 19. The LIBS system according to any one of claims 15 to 17,wherein the low-resolution spectrometer comprises a concave grating. 20.The LIBS system according to any one of claims 15 to 19, wherein thebroadband detection assembly comprises a CCD camera coupled to an outputof the low-resolution spectrometer.
 21. The LIBS system according to anyone of claims 15 to 20, further comprising probe optics defining a firstoutput light path directing a narrowband spectral portion of the plasmalight encompassing said spectral feature of the constituent element ofinterest to the high-resolution spectrometer, and a second output lightpath directing a broadband spectral portion of said plasma light to thelow-resolution spectrometer.
 22. The LIBS system according to claim 21,wherein the probe optics comprises a downstream dichroic filter centeredon the spectral feature of the constituent element of interest anddisposed to separate the plasma light into said narrowband and broadbandspectral portions.
 23. The LIBS system according to claim 21 or 22,further comprising: a first optical fiber link having a fiber inputdisposed to receive the narrowband spectral portion of the plasma lightfrom the probe optics and a fiber output connected to thehigh-resolution spectrometer; and a second optical fiber link having afiber input disposed to receive the broadband spectral portion of theplasma light from the probe optics and a fiber output connected to thelow-resolution spectrometer.
 24. The LIBS system according to any one ofclaims 15 to 23, optically coupled to a pulsed laser source generatinglight pulses apt to create a plasma upon irradiating said sample. 25.The LIBS system according to claim 24, further comprising a probe headtransportable by a user to a sample site and having a probing interfaceconfigured to irradiate the sample with the light pulses and collectresulting plasma light.
 26. The LIBS system according to claim 25,wherein the pulsed laser source is mounted within the probe head. 27.The LIBS system according to claim 25 or 26, wherein the probe opticscomprises an upstream dichroic filter centered on a wavelength of thelight pulses, the upstream dichroic filter being housed in the probehead and positioned to respectively direct: the light pulses from thelaser source towards the probing interface; and the plasma light fromthe probing interface towards the element detection assembly andbroadband detection assembly.
 28. The LIBS system according to any oneof claims 15 to 27, further comprising: a mobile housing enclosingtherein the element detection assembly and the broadband detectionassembly; a power supply unit enclosed within said mobile housing; andwire connectors providing electrical and optical communication betweenthe mobile housing and the probe head.